The tools or dies for metal processing typically are formed to close dimensional tolerances. They are massive, must be heated along with the workpiece, and must be cooled prior to removing the completed part. The delay caused to heat and to cool the mass of the tools adds substantially to the overall time necessary to fabricate each part. These delays are especially significant when the manufacturing run is low rate where the dies need to be changed after producing only a few parts of each kind.
Airplanes are commonly made from metal or composite with prefabricated parts assembled and fastened or riveted together. The labor cost for fastening is a significant cost clement and the fasteners add weight that limits overall performance and capability or adversely impacts operational costs. For military aircraft, weight translates to payload/range which is critical with modern technology where a small advantage can mean the difference between success and failure. For commercial aircraft, while weight does not translate to survivability, it is still a significant factor because the capital cost plus the operating cost are the key elements of the airline's expense. For large structure, like the wing-carry-through structure, even fastening isn't available. Instead, these structures that must be produced to close tolerance are made by electron beam welding or diffusion bonding in a hot press. Both processes are expensive and are plagued with difficulties. Electron beam welding imposes post weld stress relief requirements, weld inspection, and warpage straightening. In addition, the electron beam welding process requires expensive tooling to create closely machined joints and large "high vacuum" chambers. Hot press diffusion bonding suffers from difficulties in repeatability as well as high tooling costs, long cycle times, and inefficient energy consumption. Technologies to reduce cost and weight, accordingly, are dear in the aircraft industry. The present invention is a brazing operation using Boeing's induction heating workcell which promises significant cost and weight savings for the manufacture of large aerospace assemblies. With diffusion bonding, the entire part is heated which is problematical because it is important to hold dimensional tolerance when the complicated assembly softens. Localized heating would reduce the problems associated with heating the entire part and would conserve energy.
Commonly, in our induction heating operations, we use a retort of sealed susceptor sheets around the entire metal workpieces to control the atmosphere around the workpiece and to achieve uniform heating, as described in greater detain in U.S. Pat. No. 5,420,400 and U.S. patent application Ser. No. 08/452,216 entitled Combined Heating Cycles for Improving Efficiency in Induction Heating Operations, which we incorporate by reference. The susceptor is heated inductively and transfers its heat principally through conduction to the preform or workpiece that is sealed within the susceptor retort. While the metals in the workpiece may themselves be susceptible to induction heating, the metal workpiece needs to be shielded in an inert atmosphere during high temperature processing to avoid oxidation of the metal, so we usually enclose the workpiece (one or more metal sheets) in a metal retort when using our ceramic tooling induction heating press.
Induction focuses heating on the retort and workpiece and eliminates wasteful, inefficient heat sinks. Because the ceramic tools in our induction heating workcell do not heat to as high a temperature as the metal tooling of conventional, prior art presses, problems caused by different coefficients of thermal expansion between the tools and the workpiece are reduced. Furthermore, we are energy efficient because significantly higher percentages of our input energy goes to heating the workpiece than occurs with conventional presses. Our reduced thermal mass and ability to focus the heating energy permits us to change the operating temperature rapidly which improves the products we produce. Finally, our shop environment is not heated as significantly from the radiation of the large thermal mass of a conventional press.
We can perform a wide range of manufacturing operations in our induction heating press. These operations have optimum operating temperatures ranging from about 350.degree. F. (175.degree. C.) to about 1950.degree. F. (1066.degree. C). For each operation, we usually need to hold the temperature relatively constant for several minutes to several hours while we complete the operations. While we can achieve temperature control by controlling the input power fed to the induction coil, we have discovered a better and simpler way that capitalizes on the Curie temperature. By judicious selection of the metal or alloy in the retort's susceptor facesheets, we can avoid excessive heating. With improved control and improved temperature uniformity in the workpiece, we produce better products.
As described to some degree in U.S. Pat. No. 4,622,445 and in U.S. Pat. No. 5,410,132, we discovered an improvement for an SPF process coupling the use of ceramic dies with inductive heating. With our inductively heated SPF press or workcell, we can heat preferentially the sheet metal workpiece with induction heating without heating the platens or dies significantly and can use the ceramic dies as an insulator to hold the induced heat in the part. We can stop the heating at any time and can cool the part relatively quickly even before removing it from the die. We do not waste the energy otherwise required to heat the large thermal mass of the platens and dies. We do not force the press operators to work around the hot dies and platens. With our inductive heating workcell, we also save time and energy when changing dies to set up to manufacture different parts because the dies and platen are significantly cooler than those in a conventional SPF press. We shorten the operation to change dies by several hours. Therefore, the induction heating process is an agile work tool for rapid prototyping or low rate production with improved efficiency and versatility.
U.S. Pat. Nos. 3,920,175 and 3,927,817 describe typical combined cycles for SPF forming and diffusion bonding. Diffusion bonding is a notoriously difficult and temperamental process that has forced many SPF fabricators away from multisheet manufacturing or to "clean room" production facilities and other processing tricks to eliminate the possibility of oxidation in the bond. Oxides foul the integrity of the bond. In addition, diffusion bonds are plagued with microvoids which are difficult to detect nondestructively, but, if present, significantly diminish the structural performance of the joint. Diffusion bonding also is a time consuming process. The part typically must be held at elevated temperature and elevated pressure (about 400 psi) for several hours. For example, in U.S. Pat. No. 3,920,175, the diffusion bonding operation takes five hours at 1650.degree. F. (900.degree. C.), making the forming/bonding operation six hours. In U.S. Pat. No. 3,927,817, diffusion bonding occurs prior to forming, still requires four to five hours, and forces a six hour bonding/forming cycle at 1650.degree. F. (900.degree. C.) for the entire period. Typically a hot press diffusion bonding process for common titanium alloys used in aerospace applications will require over eight hours at 2500 psi and 800.degree. C. (1472.degree. F.), about six hours at 400 psi and 900.degree. C. (1650.degree. F.), or about two hours at 250-300 psi and 950.degree. C. (1742.degree. F.). Producing this heat and pressure for this length of time is expensive. Localized heating with higher localized pressure achieve a higher bonding force and a better bond in the process of the present invention.
The present invention is a timesaving process that promises higher quality parts at lower production costs with significant energy savings in shorter production times. The problems of hot press diffusion bonding are eliminated and more efficient manufacturing cycle is possible. Manufacturers have greater assurance in the integrity of the brazed bond so it prefer it. To achieve a satisfactory brazed bond quickly and reliably, we focus the heating on the part we are forming using an induction heater. We hold the part within insulating ceramic dies that are transparent to the time-varying magnetic field that our induction heater produces. We significantly reduce cycle time in manufacturing large aerospace parts.